The present invention relates to a new procedure for producing an aluminum alloy piston having a ferrous ring at its head end which ferrous ring has a piston ring receiving groove therein.
Aluminum alloy pistons for use in engines are well known. In certain applications where a piston ring is mounted directly in a groove formed in an aluminum piston body, excessive wear of the piston body occurs. Such wear has known adverse effects.
The art has attempted to solve the problems created by wear between the aluminum body and the piston ring by securing a ferrous metal ring on the aluminum piston and locating the piston ring in a groove formed in the ferrous metal ring. Such structures have not been entirely satisfactory because it has been difficult to secure the ferrous metal ring on the aluminum body in an effective, reliable manner.
In previous forged aluminum alloy pistons with ferrous rings, there has been a problem of maintaining the ring securely on the head end of the piston during use. This has been particularly true where the piston has been used in an internal combustion engine and subjected to temperature extremes ranging from engine operating temperature, which is generally about 200.degree. to 290.degree. C. (500.degree. to 550.degree. F.) and ambient temperature. Examples of such pistons and methods of making same are shown in U.S. Pat. Nos. 2,124,360; 3,518,741; and 3,430,969.
U.S. Pat. No. 2,124,360 discloses a method of producing an aluminum piston with a steel, bronze or ferrous alloy reinforcing band. The procedure includes preheating the piston and ring to different temperatures depending on the shrinking tension desired in the band and the stress created in the band by the expansion of the piston body while in service. The band is shrink-fitted on a reduced diameter head end of the piston blank. The reduced diameter head end is then upset forged to create a groove for the ring adjacent the head end of the piston. Temperature cycling during use tends to permit the band to loosen.
U.S. Pat. No. 3,518,741 recognizes the problem of keeping a forged-in-place reinforcing ring in firm contact with the aluminum piston body because of stresses resulting from the difference in the thermal expansion coefficients for the ring and the piston body. It also notes that if the reinforcing ring is heated to the same forging temperature as the aluminum piston body, the subsequent upset forging operation is likely to cause ring cracking. Its suggested solution to this problem is to allow the temperatures of the ring and piston head to approach each other until there exists no more than 38.degree. C. (100.degree. F.) difference between them and then coining. It also refers to a bonding process of the type disclosed in U.S. Pat. No. 2,396,730, and notes that it is rather expensive and hard to control. Its review of the prior art includes mention of U.S. Pat. No. 3,430,969 and its procedure of controlling the area-to-weight ratio of the ring to reduce the problem of looseness of the reinforcing ring on a piston.
All of these prior art patents recognize the problem of obtaining a tight and reliable fit of a ferrous or other metal reinforcing ring on a piston so that the ring will remain on the piston for a substantial time during use. The instant invention solves the problem and provides an effective method for securing the ferrous ring tightly on the aluminum piston body so that it will remain on the piston for a substantial use period.